Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to an imaging lens composed of six optical elements which is built in an image pickup device mounted in an increasingly compact and low-profile smartphone, mobile phone, PDA (Personal Digital Assistant), game console, information terminal such as a PC, or home appliance with a camera function.
In the present invention, whether or not an optical element is a lens depends on whether or not it has refractive power on an optical axis. An optical element having refractive power on the optical axis is called a lens. An optical element which does not have a lens function can contribute to improvement of aberrations in the peripheral area of an image without changing the overall focal length. In terms of lens surface shape, a convex surface or a concave surface means that the paraxial portion of the surface (the portion near the optical axis) is convex or concave. A “pole-change point” on an aspheric surface means a point on the aspheric surface at which a tangential plane intersects the optical axis perpendicularly.
Description of the Related Art
In recent years, there has been a general tendency that many information terminals have a camera function. Also, home appliances with a camera have been introduced into the market. For example, a user who is away from home can see in real time what is going on at home through the camera mounted in a home appliance by telecommunication between the home appliance and his/her smartphone and can control the various functions of the home appliance on the smartphone. This kind of home appliance is called a smart home appliance and various smart home appliances such as smart cleaners, smart air conditioners, and smart refrigerators have become widely used. On the other hand, glasses or wrist watches with a camera function as products called wearable terminals have appeared in the market. A variety of high value-added products with a function which has been inconceivable in the past are being developed by adding a camera function to existing products, and it seems that products which enhance consumer convenience or consumer satisfaction in this way will be increasingly developed in the future. The cameras mounted in such products are required not only to provide high resolution to cope with an increase in the number of pixels but also to be compact and low-profile and offer high brightness and a wide field of view. For example, for use in mobile terminals, the imaging lenses are strongly expected to be low-profile and offer high resolution and high brightness. On the other hand, for use in various home appliances, the imaging lenses are strongly expected not only to meet these expectations but also to offer a sufficiently wide field of view to capture an image of an object over a wide field of view.
However, in order to provide a low-profile imaging lens with high brightness and a wide field of view as described above, the problem related to correction of aberrations in the peripheral area of an image has to be addressed and unless the problem is addressed, it is difficult to deliver high imaging performance throughout the image.
Conventionally, for example, the imaging lenses described in JP-A-2010-026434 (Patent Document 1) and JP-A-2011-085733 (Patent Document 2) are known as compact high-resolution imaging lenses.
Patent Document 1 discloses a compact imaging lens composed of five constituent lenses in which a first positive lens, a second positive lens, a third negative lens, a fourth positive lens, and a fifth negative lens are arranged in order from an object side. The imaging lens offers brightness with an F-value of about 2 and corrects various aberrations properly.
Patent Document 2 discloses an imaging lens which includes a first lens group including a first lens having a convex surface on an object side, a second lens group including a second lens having a concave surface on an image side, a third lens group including a third meniscus lens having a concave surface on the object side, a fourth lens group including a fourth meniscus lens having a concave surface on the object side, and a fifth lens group including a fifth meniscus lens having an aspheric surface with an inflection point on the object side. This configuration is intended to provide a compact high resolution imaging lens system.
The imaging lens described in Patent Document 1, composed of five constituent lenses, corrects various aberrations properly and offers high brightness with an F-value of about 2.0 to about 2.5. However, its total track length is longer than the diagonal length of the effective imaging plane of the image sensor, which is disadvantageous in making the imaging lens low-profile. Furthermore, since its focal length is relatively long and its field of view is about 62 degrees, in order for this configuration to offer a field of view of 70 degrees or more, it is necessary to address the problem related to correction of aberrations in the peripheral area of the image.
The imaging lens described in Patent Document 2 is a relatively low-profile lens system which corrects aberrations properly. However, its F-value is 2.8 and its field of view is up to 66 degrees or so. In order for this configuration to offer brightness with an F-value of 2.4 or less and a field of view of 70 degrees or more, again it is necessary to address the problem related to correction of aberrations in the peripheral area of the image.
As mentioned above, in the conventional art, it is difficult to provide a low-profile high-resolution imaging lens which offers high brightness and a wide field of view.